Monitor and TV are different, image quality on the KS8500 is likely better due to being a VA panel so better black levels, contrast and brightness.
The UK650 is an IPS panel so lower brightness, weaker contrast but wider viewing angles and lower input lag than a older generation VA panel like on the KS8500.
IPS tends to be better played in a bright room, not a dark room as IPS panels emit a glow that gets more visible in dark environments but not everyone really notices this and it can be in the realm of nitpicking.
The main reason to use the UK650 is not because of HDR but because it has lower input lag being a PC monitor so 5-10ms compared to the 20ms of the KS8500 and the UK650 has support for Freesync.
Though it's Freesync range is limited to 40-60Hz at all resolutions, still any 60fps game will benefit as Freesync/VRR will remover judder from a game that dips into the 50's or 40's. However because the UK6xx is a Freesync v1 display it cant use HDR and Freesync at the same time, with HDR enabled Freesync wont operate on that title. Only Freesync 2 displays will work with HDR at the same time.
There's no difference between the cheaper UK600 and UK650 other than some minor extras (height adjustment stand), same IPS panel.
The UK6xx are a good affordable all rounder 4K monitor.
